Classic tactic; You use a ghost sound spell to make your Silent Image seem more life-like. It's such a standard tactic for low-level casters, that it's actually pointed out in the spell. That said, exactly how does that work? Does the use of the two spells give a bonus to the DC of one or the other, and if so what's the bonus? Or, would it make it so you have to make both will saves to believe that the illusion is fake?
I ask because I've actually come across such an elaborate version of this tactic that I'm not sure how to handle it. In a 10th level campaign, the group is helping to hold off a siege from a significantly more powerful army. When brainstorming, the rogue pops off "Well, why don't we just summon Cathulu when they show up?" Everyone laughs, then we continue planning. Except the group wizard, who reaches for the core rule book. As an illusion specialist with a back-story focused around making magical defenses for people, he has all the stuff he'd need to set it up. A combination of magical traps rigged to go off in a specific order while the cleric performs a fake summoning ritual. The spells he's using are Mindfog (for mechanical reasons, as well as the aesthetic effect), Major image (to create the greater part of the illusion), Wind wall (to create some special effects), and the big finisher Black tentacles, all set up to make it look like a portal to a dark plane is being opened, and the eldritch abomination itself is about to come through it.
How do I, as the GM, handle that? What kind of check would that involve, to disbelieve? Is there a table for determining situational-modifiers to disbelieve illusions, and what am I looking at in terms of the actual DC?
I applauded his inventiveness, and it's the last-line defence so it's not actually in the group's plan for it to be needed, but for my puposes the big-bad is supposed to come rushing in at the last minute to grab the king and run off so that the adventure can continue to the questing bit. But how is something, even a Dragon (the big-bad is an ancient Black), supposed to respond to having, literally, Cthulu summoned to stop it?
Also, I'd like to not cheap-out and say that the traps and everything just don't work right. As a 10th level wizard with all the feats and skills needed, and adequate time to set it up, it's not only unlikely that something would go wrong, but it would be an obvious GM-intervention if it did. I don't mind extending my authority to progress the plot, but I would hate to downplay with kind of creative thinking by just saying that it didn't work like planned.